1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device and a driving method thereof.
In this specification, a semiconductor device refers to any device which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all included in semiconductor devices.
2. Description of the Related Art
In recent years, attention has been driven to semiconductor devices provided with light-detecting sensor elements (also referred to as photosensors). Examples of a semiconductor device provided with a photosensor are a CCD image sensor and a CMOS image sensor. They are used in, for example, electronic devices having an imaging function like digital still cameras or cellular phones.
A semiconductor device having a photosensor in a display portion, which can detect a touch of an object to be detected (e.g., a pen and a finger) on the display portion, is also called a touch panel, a touch screen, or the like (hereinafter simply called a “touch panel”). Such a photosensor provided in the display portion enables the display portion to double as an input region; as an example, a semiconductor device having an image loading function has been disclosed in Patent Document 1.